Cervical spine rod fixation system

ABSTRACT

A cervical spine fixation system is described. The fixation system comprises a bone screw for implanting in the cervical vertebrae of the spine and for accommodating a clamping assembly. The clamping assembly comprises a rod receiving aperture which is angularly oriented a predetermined amount from perpendicular with respect to a longitudinal axis of the bone screw.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 07/998,116, filed Dec.28, 1992, now abandoned.

FIELD OF THE INVENTION

The present invention relates to instrumentation for fixation of thecervical region of the spine and more particularly to implantablecervical spine screw and rod fixation systems.

BACKGROUND OF THE INVENTION

Every year there are a significant number of people who suffer severeneck injuries from trauma. These injuries include cervical fractures,fracture dislocations combined with retropulsion of the disc and othermajor injuries. Also, each year many people undergo cervical spinesurgery for degenerative diseases, especially degenerative spinalstenosis, which involves the removal of much of the bone which supportsthe cervical region of the spine resulting in instability of thecervical region.

Treatment of these conditions has included traction either with a halteror with Crutchfield type tongs followed by application of a cast orbrace. If surgery is necessary, the area of injury is often fixed withwire to allow fusion of the vertebrae in the affected region of thevertebral column. Often the treatment includes anterior decompressionand fusion, or more recently, plates and screws have been used toimmobilize the unstable region. Such plates may be used eitheranteriorly or posteriorly, or in a few cases, both anteriorly andposteriorly.

While these fixation devices have been used effectively, they have anumber of serious disadvantages. For example, the use of wires preventsflexion, as required for effective fixation, but does not preventrotation nor extension of the cervical region, which leads to poorfixation of the vertebrae in this region. Currently, wires are nearlyalways used in combination with bone grafts. Sometimes the grafts areplaced between the posterior elements of the vertebrae in such a fashionas to prevent extension, and since the wires prevent flexion, somestabilization is obtained. However, the stabilization is usuallyinsufficient to avoid the necessity for the additional stabilityconferred by the use of halo devices, large casts or braces.

In the case where posterior plates and lateral bone mass screws areused, the stability is increased by simply screwing the plate into thebone. However, since the bone in the cervical area is relatively soft,the screws of such devices easily pull or cut out of the bone and,therefore, do not immobilize well. Even if there is no major failure ofthe device, such as the screws pulling out of the bone, the screwseventually work loose and plates, that initially prevented all motion,tend to loosen as the patient recovers from the surgery and becomes moreactive. As a result, halos, casts and braces are often used inconjunction with screw and plate fixation.

Numerous fixation devices have been described, such as those describedin U.S. Pat. Nos. 5,030,220 and 5,034,011, for use in the lumbar-sacrumregions of the spine. While these fixation devices have provensuccessful for use in the lumbar region, the physiology and structure ofthe cervical spine, C1 to C7, is very different from the lumbar-sacrumregions. For example, the bone screws, when placed in the lateral massesof the cervical vertebra, are usually placed at an angle fromhorizontal, whereas in the lumbar region they are placed in a horizontalplane. However, for proper stabilization and fixation, the hardwarecarried by the bone screws ought be such as to provide support andstabilization, generally along the axis of the cervical portion of thespine, rather than at an angle to it. It is desirable that a cervicalfixation device be developed which would preferably take into accountthese considerations. In addition, it is desirable that a cervicalfixation device be relatively compact and easy to install.

SUMMARY OF THE INVENTION

A cervical spine fixation system is described for the correction andstabilization of the spine in the cervical region. The fixation systemcomprises a bone screw wherein one end of the bone screw is implanted ina cervical vertebrae of the spine and the other end of the bone screwaccommodates a clamping assembly.

The clamping assembly comprises a rod receiving aperture angularlyoriented a predetermined amount from perpendicular with respect to alongitudinal axis of the bone screw.

In one embodiment of the present invention the clamping assemblycomprises an upper and a lower saddle which, when assembled on the bonescrew, the upper and lower saddles mate, in parallel relation, to formthe rod-receiving aperture. Means are provided for attaching theclamping assembly to the bone screw.

The attachment of the clamping assembly to the bone screw provides meansfor clamping the rod receiving aperture onto a rod located in theaperture. When installed, the clamping assemblies located on adjacentvertebrae are connected by a rod which is clamped in the rod receivingaperture, thus preventing rotation of the clamped vertebrae, relative toeach other. The installation of the system is generally performed onboth sides of a single vertebrae.

In one embodiment of the present invention a sleeve, nut which issubstantially recessed into the clamping assembly when the bone screwand clamps are assembled, is used to attach the clamping assembly to thebone screw. In another embodiment the means of attachment comprises alock washer having a plurality of spaced tangs mounted on the uppersurface of the upper saddle wherein one of the tangs of the lock washeris received in an aperture to secure the lock washer to the uppersaddle.

DESCRIPTION OF THE DRAWINGS

Features, aspects and advantages of the present invention will be morefully understood when considered with respect to the following detaileddescription, appended claims and accompanying drawings wherein:

FIG. 1 is a diagrammatic posterior view of the cervical spine;

FIG. 2 is a diagrammatic side view of the cervical spine illustratingthe cranial-ward orientation of the desired bone screw locations;

FIG. 3 is a superior sectional diagrammatic view illustrating thelateral orientation of the bone screw locations;

FIG. 4 is a perspective view illustrating the location of the bonescrews in accordance with this invention;

FIG. 5 is a perspective view illustrating the assembly of a rod on alower saddle in accordance with the present invention;

FIG. 6 is a view similar to FIG. 5 illustrating the completed assemblyin accordance with one embodiment of the present invention;

FIG. 7 is a view similar to FIG. 5 illustrating the completed assemblyin accordance with a second embodiment of the present invention;

FIG. 8 is a view similar to FIG. 5 illustrating the completed assemblyin accordance with a third embodiment of the present invention;

FIG. 9 is a view, in section, of a bone screw taken along the line 9--9of FIG. 10;

FIG. 10 is a side view of a bone screw in accordance with the presentinvention;

FIG. 11 is a plan view of the bottom surface of the lower left handsaddle;

FIG. 12 is a side view taken along the line 12--12 of FIG. 11;

FIG. 13 is a side view taken along the line 13--13 of FIG. 11;

FIG. 14 is an end view taken along the line 14--14 of FIG. 11;

FIGS. 15, 16, 17, and 18 are views similar to FIGS. 11, 12, 13 and 14but illustrating a lower right hand saddle;

FIG. 19 is a plan view of the top surface of an upper left hand saddle;

FIG. 20 is a side view taken along line 20--20 of FIG. 19;

FIG. 21 is a side view taken along line 21--21 of FIG. 19;

FIG. 22 is a side view taken along line 22--22 of FIG. 19, showing thefront face;

FIGS. 23, 24, and 25 are views similar to FIGS. 20, and 22 butillustrating an upper right hand saddle on which a lock washer issecured;

FIG. 26 is a perspective view, as seen from the front face, on the sameside as the rod receiving aperture, illustrating the mating fit of theupper and lower left saddles;

FIG. 27 is a plan view of a cervical screw spacer in accordance with thepresent invention;

FIG. 28 is a side view of the spacer taken along line 28--28 of FIG. 27;

FIGS. 29, 30, 31, and 32 are views similar to FIGS. 11, 12, 13, and 14but illustrating a lower left hand saddle with an integral spacer inaccordance with the present invention;

FIG. 33 is a side view of another embodiment of an upper saddle for usein the present invention;

FIG. 34 is a sleeve nut for use in clamping the upper saddle illustratedin FIG. 33 to the bone screw;

FIG. 35 is a perspective view illustrating an occipital plate assemblyattached to the occipital squama, on either side of the externaloccipital crest;

FIG. 36 is a plan view of a lower occipital plate;

FIG. 37 is a side view taken along line 38--38 of FIG. 36 of the plateof FIG. 36 without an occipital plate stud assembled into the loweroccipital plate;

FIG. 38 is a side view taken along line 38--38 of FIG. 36 of the plateof FIG. 36 with an occipital plate stud assembled into the loweroccipital plate;

FIG. 39 is a side view taken along line 39--39 of FIG. 36 of the plateof FIG. 36 with an occipital plate stud assembled into the loweroccipital plate;

FIG. 40 is a plan view of the under side of an upper occipital plate;

FIG. 41 is a side view of an upper and lower occipital plate assembledtogether, taken along a line similar to that of 39--39 in FIG. 36;

FIG. 42 is a plan view of an occipital plate stud; and

FIG. 43 is a side view taken along line 43--43 of FIG. 42 of the stud ofFIG. 42.

DETAILED DESCRIPTION

The system of the present invention relates to a fixation system whichcomprises bone screws, saddles, rods, clamps and nuts, all uniquelystructured for use in the cervical spine region. Fracture dislocationsand dislocations of the cervical spine are indications in which thesystem of the present invention are likely to be of most benefit. Thescrew and rod system of the present invention would prevent virtuallyall motion in the spinal segments instrumented. Two or more segments maybe instrumented and stabilized, if desired. If a patient has a severefracture of the lateral mass of a cervical vertebra, a broken segmentmay be skipped, and three segments fused together. Cross-linking may beused over the skipped segment to prevent rotation in cases of excessiveinstability. If there are multiple laminar fractures, the system of thepresent invention may be used since there is no limit to the number oflevels which can be stabilized. Attachment to the occipital bone at theskull may also be done safely and efficiently.

If anterior decompression is necessary, due to retropulsed disc materialor bone, the system of the present invention may be assembledposteriorly first, thus providing immediate fixation. The surgeon maythen proceed anteriorly, removing disc material and grafting bone asnecessary. In degenerative cervical disease, often dramatic posteriordecompression is necessary in order to decompress the nerves adequately.The system of the present invention may be used to stabilize the spineafter these procedures. In laminoplasty much of the external fixationwith casts or braces can be eliminated by use of the fixation system ofthe present invention. Usually a light neck brace or soft collar issufficient.

In addition to the foregoing, the system of the present invention may beused in the repair of a swan neck deformity. This deformity results fromextensive posterior decompression.

The system of the present invention, although intended for use primarilyin the cervical spine, may be extended into the upper thoracic spine.For example, with pathology at C7 or T1, the screws may be placed in theuppermost vertebrae of the thoracic spine and also in the cervicalspine, even though there is a rather marked curvature in this area. Therods can be contoured to fit the locations of the screws and saddles inany position or location. The applications and principles of the presentinvention are described in detail below.

FIG. 1 illustrates the cervical spine from C1 to C7. Labelled in thedrawing are the lateral mass 40, the superior facet 42, the inferiorfacet 44 and the lamina 46. FIG. 2 illustrates the angle between theouter surface 48 of the lateral mass and the transverse left to rightaxis of the cervical spine, as indicated by the dotted line 50. It isapparent that there is greater bone mass at a cranial-ward angle thanalong the transverse axis 50. Thus, in accordance with the presentinvention, the bone screws, described in detail below, are installed inthe lateral mass at a cranial-ward angle of between 20° and 30°, theangle between the transverse axis 50 and line 52. In addition, inaccordance with the present invention the bone screws are also angledlateral-ward between 10° and 20° as indicated in FIG. 3, the anglebetween dotted line 54 (representing the transverse axis front to back)and solid line 56.

FIGS. 4, 5, 6, 7 & 8 illustrate the orientation and general sequence ofinstalling the fixation system of the present invention. As illustratedin FIG. 4, bone screws 90, described in detail below, are placed in thelateral mass, angled as described above. A spacer 270, described indetail below, has also been installed to raise the bone screws to adesired elevation above the plane of the bone. Bone screws are insertedon each side of the cervical spine. Following insertion of the bonescrews, a lower saddle 110 (a lower left hand saddle) or 150 (a lowerright hand saddle), which is described in detail below, is assembledover the bone screw, as shown in FIG. 5, and a rod 58 is placed in thelower saddle. If desired a mastering procedure described in U.S. Pat.No. 4,653,481 may be used to configure the rod curvature.

The next step involves assembling an upper saddle 190 (an upper lefthand saddle) or 230 (an upper right hand saddle) over the bone screw androd, as shown in FIGS. 6, 7 & 8. The entire assembly is held together bya locking assembly 60 which operates to bolt the upper and lower saddlesonto the rod and to secure the saddles to the bone screw. Each of thecomponents of the fixation device are described in detail below.

Referring to FIGS. 9 and 10, the details of the bone screw 90 areillustrated. The bone screw and other components of the presentinvention are preferably made of 316 LVM steel, although any otherappropriate material for surgical implantation, of equivalent strength,may be used. In a preferred embodiment, the bone screw includes alongitudinal axis and a first threaded end 92 for screwing into thecervical vertebra. Adjacent to the first threaded end is a flangesection 94 and a clamp receiving section 96 is adjacent to the flange.Located on an end remote from the threaded end 90 is a second threadedend 98. The portion of the bone screw from the flange 94 to the end ofthe threaded end 98 is generally of a fixed dimension to accommodate thehalf-saddles and locking mechanism. Typically this dimension isapproximately 10 mm in length, with a diameter of about 3.5 mm. Thelength of the bone screw from the flange to the free end of the firstthreaded section 90 may vary from about 10 to 19 mm with a major threaddiameter of about 3.5 mm and a minor thread diameter of about 2.5 mm.

The flange diameter is about 5.3 mm and thus extends radially beyond theouter diameter of the threaded end 92. A portion 100 of the flangefacing the first threaded section 90 is tapered, as shown. The flange isoriented in a plane which is perpendicular to the long axis of the bonescrew. Located between the flange and the threaded end 98 is anon-threaded cylindrical section 102 which has a diameter greater thanthe diameter of the threaded section 98 but smaller than the diameter ofthe flange. The cylindrical section acts as a clamp receiving sectionfor the upper and lower half saddles. The clamp receiving sectionincludes spaced exterior flat sections 104 and 106 which mate withinternal flat sections on the lower saddle. These flat sections alsopermit the use of an appropriate tool to thread the bone screw into thelateral mass of the vertebrae.

FIGS. 11, 12, 13, and 14 illustrate a lower left hand saddle 110 whichincludes an unthreaded aperture 112. The aperture has internal flatsections 114 and 116 which mate with the flat sections 104 and 106 onthe bone screw preventing rotation of the lower saddle relative to thebone screw when the fixation system is assembled. The lower saddleincludes side faces 118 and 120, which are generally flat, a curved rearface 122, a front face 124 and top and bottom faces 126 and 128,respectively.

Located in the top face 126 and between the aperture 112 and the frontface 124 is a groove, or rod receiving half aperture, 130 on which a rodis received. The groove is preferably serrated, as indicated at 132, theserrations extending axially from one side face to the other to matewith serrations on the rod, which is described in detail below.

The bottom face 128 of the lower saddle is generally flat and abuts theflange 94 of the bone screw when assembled on the bone screw, i.e., thisface is oriented in a plane which is perpendicular to the long axis ofthe bone screw. The top face 126 of the lower left saddle is oriented ina plane which is at an angle to the bottom face 128. Thus, the axialdimension of side face 118, of about 2.1 mm, is less than the axialdimension of side face 120, of about 4.8 mm, as measured along the longaxis of the bone screw, the rear face forming a gradual transitionsection whose axial dimension progresses from that of side face 118 tothat of side face 120. Accordingly, the axial dimension from the bottomof the groove 130 to the bottom face along side face 118 is less thanthe axial dimension from the bottom of the groove 130 to the bottom facealong side face 120.

This geometry places the groove at an inclined orientation which isessentially parallel to the angular orientation of the top face of thelower saddle, i.e., the plane of the groove is angularly oriented withrespect to the long axis of the bone screw. In order to avoid sharp andbulky sections which may contact and damage the bone of the vertebrae, aportion 134 of the bottom face 128 includes a taper which is narrow atthe end adjacent to side face 118 and broader adjacent to side face 120and forms a transition section from the top face to the front face 124.As a result the front face 124 is a relatively narrow section, see FIG.14, which is angularly oriented, represented by angle "a," in the sameplane as the top face. The angle "a" is between about 10° to 25°, with12° or 22° being preferred.

In FIGS. 15, 16, 17 and 18, a lower right hand saddle 150 isillustrated. The lower right hand saddle is basically the same as thelower left hand saddle already described, except that the angularorientation is reversed. In these figures, the same numbers are used todescribe similar parts as described in FIGS. 11, 12, 13 and 14. In thisform face 118 has a greater axial length than face 120, while the topface 126, as illustrated in FIG. 18, is oriented in an opposite angularorientation as compared to face 126 of FIG. 14. Therefore, left handhalf-saddles are used for installation on the left hand side of thevertebrae and right hand half-saddles are installed on the right handside of the vertebrae, thus allowing a common angular displacement,cranial-ward, of the fixation device.

A upper left hand saddle 190, of one embodiment of the presentinvention, is illustrated in FIGS. 19, 20, 21 and 22. The upper saddleincludes a bottom face 192 adapted to be positioned in facing relationwith the top face 126 of the lower saddle, side faces 194 and 196, acurved rear face 198 and a front face 200 and a top face 202. The uppersaddle includes an unthreaded generally circular aperture 204 whichaligns with the aperture of the lower saddle and when the assembly isplace over the end 98 of the bone screw. There are no interior flats inaperture 204.

Located in the bottom face 192 and between the aperture 204 and thefront face 200 is a groove, or rod receiving half aperture, 206 on whicha rod is received for clamping. The groove 206 is preferably serrated,as indicated at 208, the serrations extending axially from one side faceto the other to mate with serrations on the rods when the fixationdevice is assembled together.

The top face 202 of the upper saddle is generally flat and is orientedin a plane which is perpendicular to the long axis of the bone screw andgenerally parallel to the lower or bottom face 128 of the lower saddle.The bottom face 192 of the upper left saddle is oriented in a planeangularly disposed with respect to the top face 202, but in an angularorientation for a mating fit with the face 126 of the lower saddle.Thus, the axial dimension of side face 196, of about 2.1 mm, is lessthan the axial dimension of side face 194, of about 4.8 mm, as measuredalong the long axis of the bone screw, the rear face 198 again forming agradual transition section whose axial dimension progresses from that ofside face 196 to that of side face 194. Accordingly, the axial dimensionfrom the bottom of the groove 206 to the top face along side face 196 isless than the axial dimension from the bottom of the groove 206 to thetop face along side face 194.

This geometry, in effect, places the groove 206 in an inclinedorientation which is essentially parallel to the angular orientation ofthe top face 126 of the upper saddle, i.e., the plane of the groove isangularly oriented with respect to the long axis of the bone screw. Inorder to avoid sharp and bulky sections which may irritate the neckmuscles of a patient when the fixation device is installed, a portion210 of the top face 202 includes a taper which is narrow at the endadjacent side face 196 and broader adjacent side face 194 and forms atransition section from the bottom face 192 to the front face 200. Ineffect, the front face 200 is a relatively narrow section, see FIG. 22,which is angularly oriented in the same plane as the top face at anangle "a". Angle "a" is between about 10° to 25°, with 12° or 22° beingpreferred.

The angular orientation of the rod receiving grooves with respect to thelong axis of the bone screws also results in a comparable angularorientation of the rod with respect to the long axis of the bone screws.Typically, the rods, made of the material already described, are about3.1 mm in diameter and may vary in length from about 25 mm to at leastabout 100 mm as required for the fixation procedure being performed. Theouter surface of the rod is serrated along its length, having forexample 28 teeth and the serrations match those on the grooves of thesaddles.

Unlike the lower saddle, the upper saddle 190 in this embodiment of theinvention has provided in the top face 202 a blind aperture 212, thelatter located between aperture 204 and the inclined portion 210 of thetop face. In one embodiment of the present invention, described indetail below, a locking assembly 60 comprises a locking washer wherein alocking tang 220 of a lock washer is received in this aperture 212. Asillustrated on an upper right hand saddle, in FIGS. 23, 24, and 25, thelock washer 214 is a separate item assembled between the upper saddle230 and a nut 260, see FIG. 7. The upper right hand saddle is basicallythe same as the upper left hand saddle already described, except thatthe angular orientation is reversed. In these figures, the same numbersare used to describe similar parts as described in FIGS. 19, 20, 21 and22.

In the assembly procedure one tang 220 of the washer is deformed intothe blind aperture, a nut 260 is threaded over the distal end of thebone screw and tightened and the remaining tangs are deformed to gripthe sides of the nut, see FIG. 26. The lock washer 214 includes threetangs, 216 and 218 which are deformed upwardly, and 220 which isdeformed downwardly and received in the blind aperture 212. Tang 220effectively attaches the lock washer 214 to the upper saddle. When tangs216 and 218 are deformed upwardly they lock the nut in place and preventit from loosening after installation.

In one embodiment of the present invention as seen in FIG. 26, once themating saddles, here the left hand saddles 150 and 190, are mounted onthe bone screw 90 and bolted together by the nut 260 and lock washer 214which, as mentioned, may be fixed to the upper saddle, as shown in FIGS.23, 24 and 25, the assembly is essentially completed. In the assembledform the tangs 216 and 218 are deformed around a nut 260 threaded on theend 98 of the bone screw. Since the lower saddle includes interior flats116 and 114 which mate with flats 106 and 104 on the bone screw, thelower saddle is prevented from rotating relative to the bone screw. Theupper saddle is prevented from rotating relative to the lower saddle dueto the inclined mating faces 126 and 192. Moreover, since the saddles,which form a rod clamp are bolted against the shoulder 94 of the bonescrew, as contrasted to being bolted against the bone of the cervicalspine, there is substantial strength in the completed assembly. Further,the rod(s) are firmly secured in the clamp formed by the mating saddleassemblies which form a rod receiving aperture since the grooves, or rodreceiving half apertures, and the rod are dimensioned for good purchaseand serrated to prevent rod rotation relative to the clamp. The above istrue of both the right and left hand configurations.

The lock nut illustrated FIG. 7 is a hexagonal nut, but other nuts maybe used. In a another embodiment of the present invention, illustratedin FIG. 6, the locking nut is an acorn nut with a smooth, curved topsurface to reduce possible irritation to the muscles which cover thefixation device when it is installed in a patient.

Occasionally, it may be necessary to raise the saddle clamp assemblyabove the level of the cervical bones to allow clearance of the bonestructure surrounding the saddle clamp assembly and to reduce the amountof bending of the rod which connects to the saddle clamps. One way ofachieving this is by the use of a cervical spacer 270 illustrated inFIGS. 27 and 28. The assembled location of the spacer is shown in FIG.4, between the tapered under surface of the screw flange 94 and thecervical bone structure. This spacer, made of material alreadydescribed, is generally cylindrical in shape and includes an aperture272 through which the bone screw passes. A counterbored section 274mates with the curved under surface 100 of the bone screw flange.Angularly oriented face 276 normally rests against the bone, the angle"b" being between 15° and 35°. The spacer provides support for theportion of the bone screw which extends above the bone and providesclearance, where desired.

A preferred embodiment of clamp assembly for providing a clearance isillustrated in FIGS. 29, 30, 31 and 32 which illustrate a lower lefthand saddle assembly 290, which in all essential respects is the same asthe lower left hand saddle of FIGS. 11, 12, 13 and 14, save for thepresence of an integral spacer extension 292. As shown, the spacer 292is located on the bottom face 128 and includes a top surface 294 whichis essentially flat and planar and oriented in a plane which isperpendicular to the long axis of the bone screw. In installation, thetop surface bears against the flange 94 of the bone screw, thus raisingthe groove 130 a distance corresponding to the vertical height 296 ofthe extension. The vertical height 296 may be varied as needed. It isunderstood that a lower right hand saddle having an integral spacer (notshown) could also be used.

In a preferred form, all parts thus far described are processed to breakall sharp edges and are electropolished and passivated. The partsdescribed may be sterilized prior to use by any of the knownsterilization procedures.

In use, the screws are located in the proper position, as determined bythe surgeon. The spacer may be placed on the bone with the bone screwpassing through the spacer. The lower saddle is assembled over the bonescrew. Several screws and saddle assemblies may be installed. If the rodis to be configured, the mastering technique referred to above may beused. The properly configured final rod is then placed in the groove(s)and the upper saddle assembly is placed over the lower saddle andsecured in place by any of the several devices already described.

In another embodiment of the present invention, illustrated in FIG. 8the cervical fixation device is designed to minimize or reduceirritation to the neck muscles which overlay the system once it has beeninstalled. In the system described above, the locking nut extends beyondthe clamping assembly. In some instances, the length of the screw-clampassembly tends to cause muscle irritation after implantation. In thisembodiment of the invention, a sleeve nut fits into a recess on theupper surface of an upper half-clamp. The novel sleeve nut in thisembodiment of the present invention greatly reduces the overall axiallength of the screw-clamp assembly, resulting in a low profile andsmooth upper surface of the fixation system which reduces the likelihoodfor the system's causing the formation of a painful bursa. The upper andlower saddle and bone screw are similar to those described above andwill only be described briefly here to point out the differences. Thesame numbers are used for parts that are unchanged from the partsdescribed previously.

The lower saddles are essentially the same as described above andillustrated in FIGS. 11-18.

An upper saddle 330 illustrated in FIG. 33, is provided with an aperture204 for receipt on the upper threaded end 98 of the bone screw 90. Theaxial length of the upper-half clamp is greater than that of the upperthreaded end 98 of the bone screw so that, when assembled, none of thethreaded portion extends beyond the upper saddle 330.

The internal diameter of the upper saddle aperture is stepped. A lowersection 332 has a diameter such that, when placed on the bone screw, theupper saddle will fit securely against the cylindrical section 102 ofthe bone screw. An intermediate section 334, which abuts the lowersection, has an intermediate diameter which is greater than the diameterof the lower section. Adjacent the intermediate section is an uppersection 336, which has a diameter larger than that of the intermediatesection. The intermediate and upper sections are separated by a chamferline 338. When assembled, the intermediate and upper sectionsaccommodate a sleeve nut 340, illustrated in FIG. 34 which is describedin detail below.

The upper saddle also includes an arm 342 in which is a groove 130,laterally of the aperture 204. The groove is serrated along its length,as indicated at 132, for mating and gripping the serrated rod. Thegeometry of the upper saddle is the same as described for the upper halfsaddles described above and places the groove at an inclined orientationwhich is essentially parallel to the angular orientation of the top faceof the lower saddle, i.e., the plane of the groove is angularly orientedwith respect to the long axis of the bone screw. When assembled, theserrated surfaces of the upper and lower saddles are in facing relationto each other and mate with, and firmly grip, the serrated rod. Whenassembled, the lower and upper saddles are placed on the bone screw, asdescribed above. A sleeve nut is used to hold the lower and uppersaddles in place on the bone screw flange 94 and to ensure a firm gripon the rod. The sleeve nut is illustrated in FIG. 34. The sleeve nut 340has an aperture 344, which is threaded so that it mates with thethreaded portion 98 of the bone screw.

The exterior of the sleeve nut is of different diameters. At the lowerend of the sleeve nut, the diameter of the sleeve nut 346 is at itssmallest and is sized such that the sleeve nut will fit into the steppedregion 334 of the upper saddle. At the upper end of the sleeve nut, andadjacent the small-diameter portion 346, is a large-diameter portion ofthe sleeve nut 348. A fillet radius 350 is located at the juncture ofthe small- and large-diameter portion of the sleeve nut. Thelarge-diameter portion is sized so that it will fit into the steppedregion 336 of the upper saddle, thus holding the upper saddle and thelower saddle securely in place when the sleeve nut is screwed onto thebone screw. The stepped interior of the upper saddle allows adistribution of the force conferred by the sleeve nut on the uppersaddle over a larger area. A chamfer line 352 is locate at the bottom ofthe sleeve nut.

The top face 354 of the sleeve nut includes four radial notches 356,placed at equal distances from each other. The notches align with prongsof a driver, not shown, so that the surgeon can more easily attach thesleeve nut to the bone screw.

In use, the lower saddle is assembled over an bone screw, and then,after the rod is in position, the upper half-clamp is installed. Thesleeve nut is then threaded on the upper threaded-end portion of thebone screw and tightened down, using a driver. The prongs of the driverare mated with the notches of the sleeve nut and the driver is then usedto tighten the sleeve nut into the upper saddle. The sleeve nut, whentightened down, is completely contained within the aperture 204, leavingexposed a small portion of the upper edge of the face 358 of the uppersaddle.

After the sleeve nut is in place, the exposed portion of the face 358 iscrimped at one point along its periphery corresponding to one of theradial notches. The crimp ensures that the sleeve nut is firmly lockedin place and that undesired rotation of the sleeve nut is inhibited.

In the event that some adjustment, and hence removal of the sleeve nutis necessary, the crimp is easily overcome by using the driver to removethe sleeve nut, and the sleeve nut is unscrewed to release the upper andlower saddles. After any required adjustment has been made, thescrew-and-clamp assembly is secured in place, as described above.

The advantage of the low profile cervical fixation system of the presentinvention is, that the axial length (height) of the screw-clamp assemblyis greatly reduced, when compared to the height of other screw-clampassemblies, since the axial length of previous screws had to be of adimension sufficient to accommodate at least one nut and locking washerabove the clamp. The height of the screw-clamp assembly as installedincreased the probability of muscle irritation by the clamp assembly.Also, the installed fixation system has a top surface which is in asingle plane which reduces muscle irritation. In one embodiment of thepresent invention, the sleeve nut fits into a recess on the uppersurface of the upper saddle, greatly reducing the overall axial heightof the bone screw above the upper saddle. This low profile of thefixation system reduces the likelihood of the system's causing theformation of a painful bursa.

In another embodiment of the present invention the cervical fixationsystem is used to treat damage to the spine in the region of the first(atlas) or second (axis) cervical vertebra. In such cases the fixationdevice is attached at one end to the cervical vertebra, as describedabove. The other end is attached to the occipital squama, on either sideof the external occipital crest, by an occipital plate assembly 350, asshown in FIG. 35. The occipital plate assembly comprises a loweroccipital plate 360 (illustrated in FIGS. 36, 37, 38, 39 and 41), anupper occipital plate 400 (illustrated in FIGS. 40 and 41), an occipitalplate stud 420 (illustrated in FIGS. 42 and 43), a nut 352, attached tothe occipital plate stud, and screws 354 attaching the lower occipitalplate to the occipital squama. A serrated rod 356 connects the occipitalplate assembly to the cervical assembly 358.

FIGS. 36, 37, 38 and 39 illustrate a lower occipital plate 360. Thelower occipital plate is a generally rectangular shaped plate 362 withan outwardly extending arm 364 perpendicular to the rectangular shapedplate, and located at the middle of one of the long sides of therectangle. The arm includes a groove 366 (see FIG. 39). The groove isserrated along its length, as indicated at 368, for mating and gripingthe serrated rod 356.

At either end of the rectangular shaped plate are apertures 370 forreceiving screws 354 and for attaching the occipital plate to theoccipital squama. Each aperture has a greater diameter 371 at an upperside of the plate that the diameter 373 at a lower side of the plate.The different diameters of the aperture allow the "V" shaped head of thescrew to seat into the aperture and to hold the lower occipital plateonto the occipital sguama.

Located between the apertures 370, adjacent to the arm, is a generallycylindrical third aperture 372 (see FIG. 37). One sidewall 374 of theaperture 372 is flattened (see FIG. 36). On an under surface of theplate the aperture is counterbored 376. The counterbored aperture 372acts to seat a threaded occipital plate stud 420. The stud has a flatsection 430 on one side for mating with the flattened sidewall ofaperture 372. The flat sides act to correctly locate the stud inaperture 372 and to prevent it rotating relative to the lower occipitalplate.

Located on the under side of the lower occipital plate are channels 378.The channels allow the lower occipital plate to flex so that it can bemolded to the gentle curve of the occipital squama and thus lay flatagainst and conform to the occipital squama surface.

Stud 420, illustrated in FIGS. 42 and 43, comprises a threaded end 422which is adjacent to a non-threaded section of similar diameter 424. Thenon-threaded section abuts a shoulder 426 which separates thenon-threaded section 426 from a larger diameter second non-threadedsection 428. The diameter of the non-threaded section 428 is dimensionedso that it will fit "snugly" into aperture 370 when the plate stud isassembled into the lower occipital plate. Non-threaded section 428includes the flat section 430, described above. Adjacent to non-threadedsection 428 is a second shoulder 432 which has an increasing tapereddiameter greater than that of the non-threaded section 428 as it extendsaway from that section. Shoulder 432 is of a larger diameter than thediameter of the aperture 372. The counterbored section of the aperture372 acts to seat the shoulder 432. The larger diameter of shoulder 432prevents the stud from slipping through the aperture 372 when the studand lower occipital plate are assembled (see FIG. 38).

The upper occipital plate 400, illustrated in FIGS. 40 and 41, is arectangular shaped plate with an aperture 402 at one end of the plate.The diameter of aperture 420 is dimensioned so that it will fit "snugly"onto non-threaded section 424 when the upper occipital plate isassembled onto the plate stud. Located at the other end of the upperplate, on an under surface of the upper plate, is a groove 404. When theupper plate is placed on the lower plate the groove 404 aligns with thegroove 368 of the lower plate to form a rod receiving aperture forsecurely gripping the serrated rod 356.

In use, the occipital plate stud is slipped into the aperture 372 in thelower occipital plate so that it extends up away from the occipitalsquama by aligning flat sections 430 and 374. The lower occipital plateis then attached to the occipital squama using screws 354. Once thelower plate is firmly attached to the occipital squama, the upper plateis placed onto the stud and aligned to bring the groves 368 and 404 infacing relation to form a rod receiving aperture. A rod is placed intothe rod receiving aperture and the upper plate is tightened to the lowerplate.

In one embodiment of the present invention the upper plate is clamped tothe lower plate by screwing an acorn nut 352 onto the threaded end ofthe stud. The acorn nut is locked into place with a lock washer 353which includes three deformable tangs 355 (only two of which are shownin FIG. 35). Two of the tangs are deformed upwardly, and one is deformeddownwardly and received in the blind aperture 406 (illustrated in FIG.41). The downwardly deformed tang effectively attaches the lock washerto the upper plate. The upwardly deformed tangs lock the nut in placeand prevent it from loosening after installation. Such a nut and lockwasher are described above in relation to FIGS. 22, 24, 25, and 26, andare identical to the nut and lock washer for attachment of the upperplate.

In an alternate embodiment, the locking assembly comprises a low profilesystem which includes a locking nut which fits into a recess on theupper surface of the upper plate. Such a low profile system is describedin conjunction with FIGS. 33 and 34, and a similar system can be usedfor attachment of the upper plate. In the case of a low profile system,the upper plate is modified to include the stepped sections (asillustrated in FIG. 33) into the upper surface of the upper occipitalplate, around aperture 402. The upper plate is then attached to theplate stud with a nut similar to that illustrated in FIG. 34.

It is apparent from the foregoing detailed description that the presentinvention has many advantages over the support systems and methods ofthe prior art as applied to the cervical spine. It will be apparent thatvarious modifications may be made to the support system and methodologyof the present invention by those skilled in the art without departingfrom the scope of the appended claims.

What is claimed is:
 1. A cervical spine fixation system comprising:abone screw, having a longitudinal axis, for implanting in cervicalvertebrae of the spine and for accommodating a clamping assembly; aclamping assembly mounted on the bone screw, wherein the clampingassembly comprises; an upper saddle comprising a rod receivinghalf-aperture angularly oriented at a predetermined fixed angle withrespect to a plane perpendicular to the longitudinal axis of the bonescrew to thereby maintain a rod received into the rod receiving aperturein generally parallel alignment with the plane of the spinal columnwherein the predetermined fixed angle ranges from about 10° to about25°; a lower saddle comprising a rod receiving half-aperture angularlyoriented a predetermined amount from a plane perpendicular with respectto the longitudinal axis of the bone screw which mates in parallelrelation with the upper saddle to form a rod receiving aperture, themating saddles further comprising an upper and lower surface on each ofthe upper and lower saddles, an aperture in the upper surface of theupper saddle, and a lock washer having a plurality of spaced tangsmounted on the upper surface of the upper saddle wherein one of thetangs of the lock washer is received in the aperture to secure the lockwasher to the upper saddle; and means for attaching the clampingassembly to the bone screw.
 2. A cervical spine fixation systemcomprising:a bone screw having a longitudinal axis, a first threaded endfor implanting into a cervical spine vertebra, a second threaded end,and a flange oriented in a plane perpendicular to the longitudinal axisof the bone screw and between the first threaded end and the secondthreaded end of the bone screw; a clamp assembly, having a generallyflat bottom face mounted on the second threaded end of the bone screwand supported by the flange, comprising a rod receiving aperture, therod receiving aperture being angularly oriented at a predetermined fixedangle with respect to the plane perpendicular to the longitudinal axisof the bone screw such that a rod is located in parallel relation to therod receiving aperture and wherein the predetermined fixed angle rangesfrom about 10° to about 25° the clamp assembly further comprising anupper and a lower saddle each of which includes parallel side faces ateither end of the rod-receiving half aperture wherein the axialdimension of a side face at one end of the rod-receiving half apertureis of a different axial dimension than the side face at the other end ofthe rod-receiving half aperture; the rod, having a longitudinal axis,and received in the rod receiving aperture; and means to secure theclamp assembly to the bone screw whereby the rod is securely fixed inthe rod receiving aperture.
 3. A cervical spine fixation systemcomprising:a bone screw comprising:a longitudinal axis; a first threadedend for implantation into a cervical spine vertebra; a clamp receivingsection including a rotation preventing means; a second threaded endspaced from the first threaded end; and a flange between the threadedends; a clamp assembly, for mounting on the bone screw, comprising:alower saddle comprising:an aperture for mounting the lower saddle on thesecond threaded end of the bone screw to abut the flange of the bonescrew wherein the aperture has a longitudinal axis which is collinearwith the longitudinal axis of the bone screw when mounted on the bonescrew; a clamp rotation prevention means in the aperture for cooperatingwith the rotation preventing means of the bone screw whereby the lowersaddle is fixed against rotation with respect to the bone screw; and arod receiving half-aperture spaced laterally from the aperture andangularly oriented at a predetermined angle with respect to a planeperpendicular to the longitudinal axis of the aperture and wherein thepredetermined angle ranges from about 10° to about 25°; an upper saddlecomprising:an aperture, with a longitudinal axis for mounting the uppersaddle on the second threaded end of the bone screw wherein the aperturein the upper saddle is alignable with the aperture of the lower saddle;and rod receiving half-aperture spaced laterally from the aperture andangularly oriented at a predetermined angle with respect to a planeperpendicular to the longitudinal axis of the aperture such that the rodreceiving half-aperture of the upper saddle is in parallel relation withthe rod receiving half-aperture of the lower saddle when mounted on thebone screw; and a rod receiving aperture, formed by the mating of thelower and upper saddles, wherein a rod received in the rod receivingaperture is in parallel relation to the rod receiving aperture andangularly oriented at the predetermined angle with respect to a planeperpendicular to the longitudinal axis of the bone screw; and means tosecure the mating saddles together and against the flange on the bonescrew whereby the rod is securely fixed in the rod receiving aperture.4. A cervical spine fixation system as recited in claim 3 wherein thesystem comprises a plurality of bone screws forming pairs adapted to belocated on each side of the cervical spine.
 5. A cervical spine fixationsystem as recited in claim 3 wherein the mating saddles further comprisea lock washer having a plurality of spaced tangs mounted on the uppersurface of the upper saddle wherein one of the tangs of the lock washeris received in the aperture to secure the lock washer to the uppersaddle.
 6. A cervical spine fixation system as recited in claim 3wherein the means to secure the mating saddles together and against theflange on the bone screw comprises a sleeve nut, wherein the sleeve nutis substantially recessed into the clamping assembly when the bone screwand clamps are assembled.
 7. A cervical spine fixation system as recitedin claim 3 wherein the system further comprises a spacer located overthe first threaded end of the bone screw.
 8. A cervical spine fixationsystem comprising:a bone screw having a longitudinal axis, a firstthreaded end for implanting into a cervical spine vertebra, a secondthreaded end, and a flange oriented in a plane perpendicular to thelongitudinal axis of the bone screw and between the first threaded endand the second threaded end of the bone screw; a clamp assembly, havinga generally flat bottom face mounted on the second threaded end of thebone screw and supported by the flange, comprising a rod receivingaperture, the rod receiving aperture being angularly oriented at apredetermined fixed angle with respect to the plane perpendicular to thelongitudinal axis of the bone screw such that a rod is located inparallel relation to the rod receiving aperture and wherein thepredetermined fixed angle ranges from about 10° to about 25° the clampassembly further comprising an upper and lower saddle each of whichincludes an upper and lower surface, an aperture in the upper surface ofthe upper saddle, and a lock washer having a plurality of spaced tangsmounted on the upper surface of the upper saddle wherein one of thetangs of the lock washer is received on the aperture to secure the lockwasher to the upper saddle; the rod, having a longitudinal axis, andreceived in the rod receiving aperture; and means to secure the clampassembly to the bone screw whereby the rod is securely fixed in the rodreceiving aperture.